if you have an external drive back up your computer (every thing) to that drive using time machine then pop in your system instal disk that came with your computer (or you retail copy of your operating system) and then restart your computer and go through the first step in the install process but instead of re-installing scroll to the top and click on disk utilities and click "Restore System from Backup" and then just choose your latest back up and you're done! took about an hour for me, i've done it many times, i'll try to find a better guide for you.

9. Navigate to the location of the backup image you want to restore (located on your external disk).

10. Click "Open" to continue.

11. Click the "Restore" button. Confirm you want to "Restore to Disk" by clicking "Restore" again. This will replace data on your Mac OS X volume with data
from your backup that has the same name and is in the same location.

12. Enter your admin name and password when prompted. If the backup disk image is encrypted, enter the disk image password if necessary. The time it takes to restore from the image depends on factors such as the amount of data on your backup disk image.

OR, if you don't have an external drive I've read you can use iDefrag (you'll have to download it) and then run disk repair via disk utility and that should work